Conventionally, in order to the number of semiconductor memory devices provided on a mother board, there has been employed many kinds of direct mounting methods on an upper and a lower surface of the mother board. Therefore, the number of semiconductor memory devices to be mounted was limited by a mounting area on the mother board.
To solve such a problem, that is, to increase the memory capacity of the mother board, there has been proposed a method disclosed in Japanese Patent Application No. 8-76947 in which semiconductor memory devices are mounted on an unit board and the unit boards are mounted on the mother board.
On the other hand, the memory capacity of semiconductor memory devices has been quadrupled along with an advancement of generations. However, when the semiconductor memory devices are replaced with new generation semiconductor memory devices (namely, memory devices which have four times larger memory capacity than the previous generation semiconductor memory devices), the number of terminals for connecting the unit board with the mother board and an allocation thereof are changed. Therefore, the number of terminals and the allocation thereof on the mother board are required to be changed each time the generation of the semiconductor memory devices changes in the aforementioned method.